UNITED NATIONS (AP) - U.N. Secretary-General Ban Ki-moon warned Thursday that a new peacekeeping force for Darfur could fail unless it gets 24 critically needed helicopters and he appealed to all countries for help.

Ban urged U.N. Security Council members to use their influence to turn international concern for Darfur into concrete offers of aircraft. "While helicopters alone cannot ensure the success of the mission, their absence may well doom it to failure," he said in a letter to Council members.

With only three weeks left before the 26,000-member U.N.-African Union force is scheduled to start deploying, Ban lamented the U.N.'s failure to get a commitment for even one helicopter.

Ban said he had personally contacted every country with the potential to contribute a helicopter - from industrialized to major developing nations - "to no avail."

He said he was sending two high-level envoys to a summit of European Union and African leaders in Lisbon, Portugal, this weekend "to directly engage with as many key leaders as possible on this subject."

"We are at the critical moment for Darfur," Ban said. "Member states have spoken clearly about what must be done. It is time for them to walk their talk."

The conflict in Sudan's western Darfur region has claimed more than 200,000 lives and uprooted 2.5 million people since ethnic African rebels took up arms against the Arab-dominated government in 2003. Critics accuse Sudan of arming the janjaweed Arab militias that have terrorized Darfur villages - a charge Khartoum denies.

Ban said the 18 transport helicopters and six light tactical helicopters are essential for "an effective, robust force."

"Without effective, efficient mobility and capacity, we will not be able to protect the civilians and even our own soldiers," he said. The U.N. chief stressed that rebel leaders would not join the peace process without an effective peacekeeping force.

"But for this we need on-the-ground capability - specifically helicopters. We are not getting them. Because of that the entire mission is at risk," Ban said.

The U.N. assistant secretary-general for peacekeeping, Edmond Mulet, and his deputy chief of staff, Kim Won-soo, were dispatched to the European Union-African summit in Portugal. They will also press Sudan's leaders to accept non-Africans in the AU-U.N. peacekeeping force, Ban said.

At Sudan's insistence, the U.N. Security Council agreed the force would be predominantly African. But the Sudanese government has refused to approve non-African units from Thailand, Nepal and Nordic countries.

Ban said he would send a personal letter to Sudanese President Omar al-Bashir with the envoys.

Sudan's U.N. ambassador, Abdalmahmood Abdalhaleem Mohamad, said the U.N. delegation will meet the Sudanese foreign minister and if time permits, the president. He insisted Sudan was committed to "dialogue and transparency" and would "deliver on our commitments."

In a letter to Ban obtained by The Associated Press, the Sudanese ambassador addressed some problems raised last week by U.N. peacekeeping chief Jean-Marie Guehenno, who warned that Sudan was putting up numerous obstacles that could destroy the force's effectiveness.

Mohamad did not address the dispute over the force's composition, but he said the issue of land for the force's operations has been resolved and the government had approved night flights for medical evacuations.

Guehenno had criticized Sudan for proposing a Status of Forces Agreement that "would make it impossible for the mission to operate."

The agreement includes a proposal to allow Sudan's government to "temporarily disable the communications network" during security

operations to protect the country's sovereignty. Another proposal would require the peacekeeping force to provide "advance notification to the government for all staff, troop and asset movements," Guehenno said.

Mohamad said the agreement was under discussion.

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